Three dimensional (3D) laser processing machines include a laser processing head having a focusing element and a laser beam guide, as disclosed, for example, in EP 1 640 105 A1. In some cases, the focusing element is a mirror and the beam guide is open towards the processing location. Such machines are mainly used for laser welding.
A clean and constant atmosphere is necessary in the beam guide. Fluctuating CO2 content, the presence of other substances which influence the beam spread, or an uneven temperature distribution over the beam cross-section can change the unprocessed beam and consequently also the properties at the focal point so that the constancy of the processing operation becomes worse.
A solution which is already known is to fill the beam guide with clean nitrogen and to obtain, with a given volume flow, a throughput and also an elevated pressure counter to the introduction of contamination from the environment.
Since the known laser processing head is open towards the processing location, a large amount of expensive pure gas is discharged from the beam guide in this solution.
Similar problems also occur in laser cutting assemblies having optical mirror units, such as, for example, as disclosed in EP 1 623 790 A1. The laser processing head of this laser cutting assembly does not have a lens which seals the beam guide in the direction towards the laser processing region, but instead an open aperture for separation. Expensive pure gas is also discharged from the beam guide through that aperture, but in a substantially smaller quantity than in the above-described laser welding assembly.
EP 1 182 002 A1 has disclosed, for laser cutting machines having an optical lens unit, flushing a lens at the beam guiding side with very pure gas and accordingly increasing the pressure of the flushing gas in the beam guide when pressure decreases. In this case the beam guide is not constructed so as to be open towards the workpiece because the lens seals the beam guide and the processing space relative to each other. Therefore, the pressure adjustment can be carried out in this instance with very pure (expensive) gas without this resulting in a serious cost disadvantage.